As is conventionally known, in a diesel engine used as an automobile engine or the like, fuel injection control is performed that adjusts a period and amount of fuel injection from a fuel injection valve (also referred to below as an injector) according to engine revolution, amount of accelerator operation, coolant temperature, intake air temperature, and the like (e.g., see Patent Citation PLT 1 listed below).
Diesel engine combustion is achieved by premixed combustion and diffusion combustion. When fuel injection from a fuel injection valve begins, first a combustible mixture is produced by the vaporization and diffusion of fuel (ignition delay period). Next, this combustible mixture self-ignites at about the same time at numerous places in a combustion chamber, and combustion rapidly progresses (premixed combustion). Further, fuel continues to be injected into the combustion chamber, so that combustion is continuously performed (diffusion combustion). Thereafter, unburned fuel exists even after fuel injection has ended, so heat continues to be produced for some period of time (afterburning period).
Also, in a diesel engine, as the ignition delay period grows longer, or as the vaporization of fuel in the ignition delay period grows more intense, a flame propagation speed after ignition rises. When this flame propagation speed is high, the amount of fuel that burns at once becomes too great, pressure inside the cylinder drastically increases, thus generating vibration or noise. Such a phenomenon is called diesel knocking, and often occurs particularly when operating with a low load. Also, in this sort of situation, a drastic elevation in combustion temperature is accompanied by an increase in the amount of nitrogen oxide (referred to below as “NOx”) produced, and thus exhaust emissions become worse.
In view of this, various fuel injection control apparatuses have been developed in order to prevent this sort of diesel knocking and reduce the amount of NOx that is produced. For example, ordinarily fuel injection from a fuel injection valve is performed intermittently by dividing this injection into a plurality of instances.
For example, in Patent Citation PLT 2 listed below, the injection period of pilot injection, in which fuel is injected prior to main injection, is set to be earlier by an amount equal to a delay period from the pilot injection period to the actual ignition period. As a result, the ignition period of fuel injected in the pilot injection is substantially the same as the main injection period, thereby reducing the amount of PM (Particulate Matter) and HC (Hydrocarbons) that are emitted.